I think she's straddling his thigh. If you are in possession of a clitoris, you can stimulate it in a pretty awesome way like that.

Well, okay, actually it would probably work a lot better if he were on his back with that leg pulled up a bit and she were lying on top of him.

But still: the thing they're doing is totally a thing. (TMI warning) My ex-boyfriend loved it for some reason. Popular among lesbians, I'm told (rubbing your clit against someone's thigh, that is. My ex has only attracted one self-identified lesbian, AFAIK).